Satellite mobile communication is one of the necessary means for communication anywhere. In recent years, the fourth generation (4G) terrestrial cellular mobile communication became matured gradually, and the 3rd Generation Partnership Project Long Term Evolution (3GPP-LTE), which is based on key techniques such as Orthogonal Frequency Division Multiplexing (OFDM), Multiple-Input Multiple-Output (MIMO), and same-frequency networking, etc., has been commercially available gradually. How to apply Terrestrial LTE (T-LTE) that has characteristics such as high speed, high capacity, high spectrum efficiency, and high power efficiency, etc. in satellite mobile communication to establish multi-beam Satellite LTE (S-LTE) mobile communication systems based on same-frequency networking technique is a hotspot and a challenge in the current research in satellite mobile communication field.
Providing mobile communication services with Geostationary Earth Orbit (GEO) satellites has many advantages: theoretically, a single GEO satellite can cover 42.2% earth surface, and 3 GEO satellites can cover the global regions except for the south and north poles; the signal transmission delay is a constant; frequent inter-satellite switchover is unnecessary; the Doppler shift is small; the technology is matured relatively, and the investment risk is lower, etc. However, a satellite mobile communication system that solely employs GEO satellites has some problems: (1) the GEO satellites have been occupied densely; (2) the south and north poles can't be covered; (3) the signal transmission distance is longer in middle and high latitude regions, since the communication elevation angle thereof is low; (4) high constructions and mountains, etc. between the ground terminal and the satellite hinder the signal transmission, and thus cause shadow regions, so that it is difficult to realize global coverage of satellite mobile communication. For Inclining Geostationary Synchronized Orbit (IGSO) satellites, the sub-satellite track is in a “8” shape with the equator as the symmetry axis, since the orbit inclination angle is greater than 0°; the larger the orbit inclination angle is, the larger the “8”-shaped region is. Therefore, IGSO satellites can effectively overcome one drawback of GEO satellites that the elevation angle in middle and high latitude regions is always low. However, the covering capacity of an IGSO satellite is inferior to that of a GEO satellite. When GEO satellites and IGSO satellites are utilized in combination to complement each other in a network, global covering can be realized.
A multi-beam GEO-IGSO S-LTE mobile communication system employs a large-size antenna array on the satellite to produce multiple beams, which form multiple cells when they reach the ground surface. Similar to T-LTE, GEO-IGSO S-LTE also employs same-frequency networking pattern, in which different main synchronization sequences are configured for adjacent cells. However, GEO-IGSO S-LTE has severe Inter-Beam Interference (IBI), and the overlap range between adjacent cells is large. To implement better cell searching, more main synchronization sequences are required for GEO-IGSO S-LTE.